Diphenylsulfone cross-linking compounds represented by the following formula (1) are known as a color-developing composition or an image storage stabilizer (e.g., see Patent Document 1).
[wherein X and Y may each be different, and each either represents a linear or branched C1-C12 hydrocarbon group which may be saturated or unsaturated or may have an ether bond, or represents the following formula
(wherein R′ represents a methylene group or ethylene group; T represents a hydrogen atom or C1-C4 alkyl group); R1-R6 each independently represents a halogen atom, C1-C6 alkyl group or C2-C4 alkenyl group; m, n, p, q, r and t each represents an integer of 0 to 4 and when representing an integer of 2 or more, R1-R6 may each be different; and “a” represents an integer of 0 to 10)
such composition is a mixture of, for example, diphenylsulfone cross-linking compounds with different degrees of polymerization that are produced by reacting 4,4′-dihydroxydiphenylsulfone with bis(2-chloroethyl)ether. Therefore, the raw material 4,4′-dihydroxydiphenylsulfone is remained in the reactant.
However, since the 4,4′-dihydroxydiphenylsulfone referred to above is currently designated as a Type II Monitoring Chemical Substance set forth in the “Law Concerning the Examination and Regulation of Manufacture, etc. of Chemical Substances”, compositions with a low content of this compound has been longed for. Further, this composition has been insufficient in terms of heat resistance at the background part when used for a recording material.    [Patent Document 1] Japanese Laid-Open Patent Application No. 10-29969